The University of Florida Theoretical Astrophysics Seminar is held on Wednesdays at 1:45pm. During the 2020-21 academic year, all seminars will be held virtually via Zoom.

Spring 2021 Schedule

February 10: Thankful Cromartie (Cornell University)

Title: A Space-Based Physics Lab: Probing Neutron Star Physics & Gravitational Waves with Millisecond Pulsar Timing


Millisecond pulsar (MSP) timing - the process of accounting for every rotation of a rapidly spinning neutron star over long time spans - is a powerful tool for probing realms of physics that are otherwise inaccessible to Earth-based scientists. In this talk, I will discuss how the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array uses pulsar timing for the detection of low-frequency gravitational waves. Additionally, I will discuss our use of relativistic Shapiro delay measurements to precisely determine MSP masses. The pulsar J0740+6620 is one of the 70+ MSPs timed by NANOGrav. By combining NANOGrav observations of this source with orbital phase-specific observations near conjunction using the Green Bank Telescope, we measured the pulsar's mass to be ~2.14 +/-0.09 solar masses. The measurement of such a massive neutron star is a significant constraint on the poorly understood neutron star equation of state.

March 3: Steve Taylor (Vanderbilt University)

Title: New results from the Pulsar Timing Array hunt for nanohertz-frequency gravitational waves


Supermassive black holes lurk at the heart of massive galaxies. These titans form binaries over cosmic time as a byproduct of galaxy growth, emanating gravitational waves in the nanohertz-frequency sensitivity band of networks of precisely timed pulsars. Pulsar-timing arrays (PTAs) like the North American Nanohertz Observatory for Gravitational waves (NANOGrav) and the International Pulsar Timing Array are poised to chart this new frontier of gravitational wave discovery within the next several years. I will present new results from NANOGrav's most recent search, discuss some milestones on the road to the exciting next decade of PTA discovery, and highlight new techniques to facilitate future multi-messenger supermassive black-hole binary characterization.

March 17: Jorge Moreno (Pomona College)

Title: Spatially resolved galaxy interactions


The focus of this talk is to address the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations, which employs the "Feedback in Realistic Environments" model (FIRE-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encourters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially resolved (integral-field spectroscopic) galaxy surveys -- capable of examining vast and disverse samples of interacting systems -- coupled with multi-wavelength campaigns aimed to capture their internal ISM structure. If time allows, I will also speak about dark-matter free dwarf galaxies in FIREBox, a cosmological simulation of galxy formation with FIRE.

March 24: [provisionally reserved for faculty search]

March 31: [provisionally reserved for faculty search]

April 7: [provisionally reserved for faculty search]

April 14: [provisionally reserved for faculty search]

Fall 2020 Schedule

September 30: Karan Jani (Vanderbilt University)

Title: GW190521 and Multi-Band, Multi-Messenger Astronomy


The new gravitational-wave signal GW190521 in LIGO and Virgo marks the first observational detection of the elusive intermediate-mass black holes. In this talk, I would review the implications of this historic discovery to cosmology and pair-instability supernovae theory. Further, I would highlight how this discovery opens a unique science case for a joint multi-band, multi-messenger gravitational-wave astronomy with the upcoming ESA/NASA space mission LISA and a potential gravitational wave experiment on the Moon.

October 7: Wenbin Lu (Caltech)

Title: A Unified Picture of Fast Radio Bursts


Fast radio bursts (FRBs) are short duration (~ms), very bright radio transients. Their detection a decade ago was a major unexpected discovery in astronomy in decades. Hunting for FRBs and measuring their physical properties have become one of the leading scientific goals in astronomy. This effort has led to a rapidly growing sample with extremely diverse properties in luminosity (10^38 to 10^45 erg/s), duration (0.1 ms to 10 ms), and repetition rate (some objects have multiple bursts in an hour and many just one burst in a few years). I will present a study of their cosmological volumetric rate density and provide evidence that these bursts all belong to the same class of transients --- most likely all are repeaters. According to my model, disturbances close to the surface of a magnetar launch Alfven waves into the magnetosphere, which propagate to a distance of a few tens of neutron star radii and then produce coherent radio emission. The coincident hard X-rays associated with the Galactic FRB 200428 can be understood in this scenario. This model provides a unified picture for weak Galactic FRBs as well as the bright bursts seen at cosmological distances. If time allows, the polarization properties of FRBs will also be addressed.

October 14: Maya Fishbach (University of Chicago -> Northwestern)

Title: Astrophysical Lessons from Gravitational-Wave Populations of Black Holes and Neutron Stars


LIGO/Virgo detected eleven systems of merging black holes and neutron stars in their first two observing runs, with several additional events published from the third observing run so far. Studying the masses, spins and redshifts of these sources reveal several exciting features, including gap(s) in the mass spectrum, a preference for equal-mass pairings, and signs that the merger rate evolves with redshift. I will discuss these features, and how recent "exceptional" events from LIGO/Virgo fit into the population. These population properties may reveal how black holes are made, as well as providing insight into nuclear physics and cosmology.

October 21: Kayhan Gultekin (University of Michigan)

Title: Supermassive Black Hole Pairs


Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on the black hole outflows. Then I will concentrate on my group's recent work searching for dual and binary supermassive black holes along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binaries.

October 28: Michela Mapelli (University of Padova)

Title: The multifaceted formation of binary black holes


The latest results from the LIGO-Virgo collaboration challenge the astrophysical scenarios of black hole formation. In this talk, I will discuss the main astrophysical formation channels of binary black holes. On the one hand, models of stellar evolution and pair instability supernovae suggest a gap in the mass spectrum of black holes between ~60 and ~120 Msun. The boundaries of this gap depend on stellar rotation and on the efficiency of envelope removal. On the other hand, extreme dynamical processes in dense star clusters can fill the mass gap, via multiple stellar collisions and dynamical exchanges. Moreover, stellar dynamics enhances the formation of black hole -- neutron star systems with extreme (<1:10) mass ratios. Based on a data-driven model, I will discuss the merger history of dynamical versus isolated binary compact objects across cosmic time, and its dependence on the cosmic star formation rate and on the stellar metallicity.

November 11: Mohammad Safarzadeh (UC Santa Cruz)

Title: The Astrophysical Context of Gravitational Wave Events


We live in an era of breakthrough discoveries in gravitational waves (GW) astronomy. Every month or so, such discoveries by LIGO/Virgo have been making headlines because these events' nature has been far from the expectations. But why are we puzzled? And what is the road ahead for us to a deeper understanding? I discuss two of the puzzling events that LIGO has discovered: 1) The most massive binary black hole merger with masses above the pair-instability limit. After an overview of the physics of pair-instability supernova, I will discuss how a broader perspective on the host environment of binary black holes can hold the key to understanding the nature of such massive systems. 2) The most massive binary neutron star merger. I will present possible scenarios to explain why we have not detected such systems in the radio observations before and how the key to understanding these events might lie in the r-process enrichment in the early universe and magnetic field evolution of neutron stars. Through these two examples, I will conclude that the synergy of electromagnetic-wave astronomy with gravitational wave astronomy is essential for gaining insights into the surprises that are being uncovered with these new observations.

November 18: Katelin Schutz (MIT)

Title: Making dark matter out of light: the cosmology of sub-MeV freeze-in


Dark matter could be a "thermal-ish" relic of freeze-in, where the dark matter is produced by extremely feeble interactions with Standard Model particles dominantly at low temperatures. In this talk, I will discuss how sub-MeV dark matter can be made through freeze-in, accounting for a dominant channel where the dark matter gets produced by the decay of plasmons (photons that have an in-medium mass in the primordial plasma of our Universe). I will also explain how the resulting non-thermal dark matter velocity distribution can impact cosmological observables.

November 25: [No seminar; Thanksgiving week]

December 9: Gongjie Li (Georgia Tech)

Title: Dynamical Origin and Habitability of Planets -- On the Spin-Axis Variations of Stars and Planets


The spin-axis dynamics of stars reveal histories on the formation mechanism of planetary systems. For instance, the observed spin-orbit misalignments suggest different pathways on the migration of planets. On the other hand, the variation of a planet's obliquity plays an important role in determining its climate, and the existence of massive satellites can influence such variations. For instance, the Earth's obliquity is stabilized by the Moon, and would undergo chaotic variations without the Moon. In this talk, I will discuss the origin of the ultra short period planets. Then, I'll present a simplified perturbative approach, and demonstrate that without the Moon, the stochastic change in the Earth's obliquity is sufficiently slow to not reach high values (>40 degrees) in billion-year timescales. In the end, I will discuss obliquity variations of exoplanets.

Other Events

Students may receive credit for attending the Astrophysics Seminar by registering for PHY 6391.